In the German Pat. No. 2,162,572 a compressed air cylinder is disclosed which accommodates a reciprocating piston guiding a transmission element which projects beyond the cylinder through a respective longitudinal slot in the cylinder barrel. The slot is sealed by a seal band which is made of magnetizable steel. Arranged along the cylinder are a plurality of magnets which retain the seal band in position and prevent a sagging thereof into the internal space of the cylinder. The use of magnets renders this cylinder complicated and still does not provide a satisfactory sealing of the longitudinal slot towards the outside so that dirt is not prevented from penetrating into the interior.
It has thus been proposed by the German Pat. No. 3,124,878 to provide a shuttle piston cylinder with an inner and an outer seal band of elastomer material. The inner band is provided with a longitudinal rib whose upper portion is wedge-shaped and engages in a longitudinal wedge-shaped groove of the outer band. During movement of the piston e.g. to the left, the rib is continuously disengaged from the groove at the left side of the piston while at the right side thereof is pushed into the groove. A drawback of this prior art is the use of two seal bands in order to provide a sealing also towards the outside. Moreover, the seal bands are made of elastomer material so that their strength and stability are relatively low.